In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is slowly starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drive trains into their vehicle line-ups. To meet consumer expectations, however, the automobile industry must not only achieve a greener drive train, but must do so while maintaining reasonable levels of performance, range, reliability, safety and cost.
The most common approach to achieving a low emission, high efficiency car is through the use of a hybrid drive train in which an internal combustion engine (ICE) is combined with one or more electric motors. While hybrid vehicles provide improved gas mileage and lower vehicle emissions than a conventional ICE-based vehicle, due to their inclusion of an internal combustion engine they still emit harmful pollution, albeit at a reduced level compared to a conventional vehicle. Additionally, due to the inclusion of both an internal combustion engine and an electric motor(s) with its accompanying battery pack, the drive train of a hybrid vehicle is typically much more complex than that of either a conventional ICE-based vehicle or an all-electric vehicle, resulting in increased cost and weight. Accordingly, several vehicle manufacturers are designing vehicles that only utilize an electric motor, or multiple electric motors, thereby eliminating one source of pollution while significantly reducing drive train complexity.
In order to achieve the desired levels of performance and reliability in an electric vehicle, it is critical that the temperature of the traction motor remains within its specified operating range regardless of ambient conditions or how hard the vehicle is being driven. A variety of approaches have been used to try and adequately cool the motor in an electric car. For example, U.S. Pat. No. 6,191,511 discloses a motor that incorporates a closed cooling loop in which the coolant is pumped through the rotor. A stationary axial tube mounted within the hollow rotor injects the coolant while a series of blades within the rotor assembly pump the coolant back out of the rotor and around the stator. Heat withdrawal is accomplished using fins integrated into the motor casing that allow cooling via ambient air flow.
U.S. Pat. No. 7,156,195 discloses a cooling system for use with the electric motor of a vehicle. The refrigerant used in the cooling system passes through an in-shaft passage provided in the output shaft of the motor as well as the reduction gear shaft. A refrigerant reservoir is formed in the lower portion of the gear case while an externally mounted cooler is used to cool the refrigerant down to the desired temperature.
U.S. Pat. No. 7,489,057 discloses a rotor assembly cooling system utilizing a hollow rotor shaft. The coolant feed tube that injects the coolant into the rotor shaft is rigidly coupled to the rotor shaft using one or more support members. As a result, the rotor and the injection tube rotate at the same rate. The coolant that is pumped through the injection tube flows against the inside surface of the rotor shaft, thereby extracting heat from the assembly.
While there are a variety of techniques that may be used to cool an electric vehicle's motor, these techniques typically only provide limited heat withdrawal. Accordingly, what is needed is an effective cooling system that may be used with the high power density, compact electric motors used in high performance electric vehicles. The present invention provides such a cooling system.